find Polymorphic Serializer Or Null
open override fun findPolymorphicSerializerOrNull(decoder: CompositeDecoder, klassName: String?): DeserializationStrategy<out DateTimeUnit.DateBased>?
open override fun findPolymorphicSerializerOrNull(encoder: Encoder, value: DateTimeUnit.DateBased): SerializationStrategy<DateTimeUnit.DateBased>?